how to bend straight faces to make a curve
There are different ways (as always) to try and get what you want to achieve.

To prepare I rotated the Mesh 90° around the X axis and applied the rotation. So we can watch it from Orthograpic Top View while editing (I just prefer that).


Like @AntonPg suggested:


Go into Edit Mode
Select the middle Edge
Activate Proportional Editing (experiment with the settings ..)
Press G and then Shift + Z to move the selected Edge only on the X and Y axis (without Z axis). Then move and adjust the proportional editing influence radius (MouseWheel).